1. Field of the Invention
The present invention relates to a frequency dividing circuit, and in particular, relates to a frequency dividing circuit suitable for high speed operations.
Priority is claimed on Japanese Patent Application No. 2006-317022, filed Nov. 24, 2006, the content of which is incorporated herein by reference.
2. Description of Related Art
FIG. 4 is a block diagram of the conventional frequency dividing circuit E′. In FIG. 4, output from the Q terminal of a D-type flip flop (hereafter referred to as “DFF”) 3′ is fed back to the D input terminal through a variable delay element 1 and an OR gate 2. The delay time at this variable delay element 1 is controlled by the control signal 101. The output of AND gate 6 is input to the other input terminal of the OR gate 2. The output of this AND gate 6 is the logical product of the Q output of DFF 4 and a signal that is the QB output of DFF 4 delayed by the delay element 5. The output of this AND gate 6 is input to the D input terminal of DFF 3′ through the OR gate 2.
The frequency dividing circuit E′ outputs a frequency-divided signal 104 synchronized with the input clock 102. More specifically, while clock 102 is input to the clock terminals of DFF 3′ and DFF 4 respectively, the start signal 103 is input to the CD terminal of DFF 3′ and the D input terminal of DFF 4. As a result, the Q output and the QB output of DFF are generated. Subsequently, the Q output of DFF 4 and the signal, that is QB output of DFF 4 delayed by the delay element 5, are input to the AND gate 6. The result is that positive pulse 105 is generated synchronized with the input clock 102 from the AND gate 6, and this positive pulse 105 is supplied to the D input terminal of DFF 3′ through the OR gate 2.
When clock 102 is input to the clock terminal of DFF 3′, the frequency dividing operation starts, triggered by the positive pulse 105. As a result, the frequency-divided signal 104 of the clock 102 is output from the inverted output QB terminal of DFF 3′. The output pulse of the Q terminal of DFF 3′ is delayed by a specific time set by the control signal 101, is fed back to the D input terminal of DFF 3′, and the frequency dividing operation of DFF 3′ is continued. In this way, the frequency-divided signal 104 of the clock is output from the output terminal of DFF 3′.
The frequency dividing circuit E′ that outputs the frequency-divided signal 104 synchronized with the input clock 102 is provided with DFF 3′, variable delay element 1 and DFF 4. Clock 102 is input to the clock terminal of this DFF 3′, and the inverted output QB terminal of the DFF 3′ outputs the frequency-divided signal 104 of clock 102. The variable delay element 1 delays the output pulse of the non-inverted Q output terminal of the DFF 3′ by the specified setting time, and feeds it back to the D input terminal. DFF 4 is required to generate pulse 105 synchronized with the clock 102 at the start of the frequency dividing operation (see Japanese patent publication No. 2055380 (page 3 and in FIG. 1)).
However, the conventional frequency dividing circuit is not provided with means for detecting the relationship between the edge of the signal fed back to the D input terminal through the variable delay element 1 and the timing of the edge of clock 102. For this reason, when the timing relationship does not satisfy the setup/hold time of DFF 3′, then the DFF 3′ causes a meta-stable phenomenon, and the frequency division output becomes unstable. As is already known, meta-stable phenomenon refers to the unstable condition of the output signal when the setup time or the hold time is not maintained in the latch or flip-flop input signal. This meta-stable phenomenon varies from several tens of ps to several ns in standard logic, and it becomes critical jitter in high speed operations at about 40 GHz.
In addition, when the frequency of clock 102 changes, the meta-stable phenomenon may occur. For this reason, a control signal corresponding to the frequency of clock 102 should be used.
Generally, the variable delay element 1 may generate the meta-stable phenomenon when the delay varies with the temperature. For this reason, a control signal calibrated with the temperature variation must be used.
Thus, it is difficult to obtain a stable and low jitter frequency-divided signal in high speed operations of about 40 GHz.